P0420 Catalyst System Efficiency Below Threshold
hi need help basically driving along and the engine light came on but car was still running ok, hooked it up to a fault reader got the following code <P0420 Catalyst System Efficiency Below Threshold >once the code was reset the car stalled after it was started a few times plus the idle wasnt brill plus alot of pops and bangs when the car is reved slightly so basically over fuelling, and ideas ? put a multimeter on the front lamda sensor where it plugs in and gettin a stable 3.8v does that mean it knackered ? car has a full decat 3" system and a blitz dump valve dunno if this would cause this , its an 2002 sti, cheers in advance for the help